Spindle type veneer peeling lathes conventionally incorporate backup rolls for counteracting the forces imparted to the peel block by the veneer knife as it peels veneer from the block. The backup rolls are positioned against the block, about 90 to 180 degrees of arc away from the knife, and forced against the rotating surface of the block to stabilize the block and preventing it from bowing or lifting away from the knife during the peeling operation.
Early backup rolls were not powered, but merely consisted of a pair of idler rolls which spun freely against the rotating block during the peeling operation. However, idler backup rolls were found to impart significant drag forces to the rotating block, resulting in increased horsepower requirements for driving the lathe spindles to maintain proper peeling of the block. Accordingly, the art evolved powered backup rolls which are drivingly rotated against the block as it is peeled. The driven (or "powered") backup rolls thus contribute to the driving rotation of the block, obviating the need for increased horsepower in the lathe spindle drive motors.
Prior art powered backup rolls have had several configurations, one of which is exemplified by Canadian patent No. 1,166,045 issued Apr. 24, 1984 for an invention of Byron B. Brookhyser et al. entitled "Veneer Lathe Apparatus and Method Using Independently Adjustable Powered Backup Roll". It will be noted that only one of Brookhyser's backup rolls is powered--the second backup roll idles against the block (that is problematic, because the idling roll tends to drag against the block unless its rotational speed is perfectly matched to that of the block, which is very difficult to achieve in practice). Brookhyser's backup rolls are each independently pivotally positionable about a separate pivot axis. Accordingly, Brookhyser provides a first hydraulic cylinder for pivotally positioning the first backup roll with respect to its pivot axis and a second hydraulic cylinder for pivotally positioning the second backup roll with respect to its pivot axis. The two backup rolls and their respective positioning cylinders are in turn mounted on an assembly which is pivotally positioned with respect to the lathe frame by a third hydraulic cylinder. Brookhyser thus requires a rather complicated control system capable of independently actuating each of the three hydraulic cylinders to maintain the two backup rolls in position relative to the peel block.
Another prior art configuration is exemplified by Canadian patent No. 1,202,548 issued Apr. 1, 1986 for an invention of Leonard L. Hayes entitled "Veneer Lathe Drive with Powered Rolls". Hayes' system incorporates three powered drive rolls. A first "reference" drive roll engages the underside of the block and is locked in a predetermined reference position to serve as a reference surface for the block as it is pressed into contact with the reference roll by the other two drive rolls which are mounted on a common support so that they remain a fixed distance apart. The common support on which the other two rolls are mounted is pivotally rotatable about an axis of the lathe frame. Theoretically, the three powered rolls optimally track the block's decreasing diameter as veneer is peeled therefrom. However, it is expected that serious control problems would be encountered in moving Hayes' relatively massive structure through its various operational positions at the speeds demanded of state of the art veneer lathes.
The prior art configurations aforesaid are mechanically complex in that they each incorporate a large number of moving parts. Movement of each part is governed by a separate control element. The various control elements each have inherent positioning errors. The overall configuration is only able to position the backup roll to within a margin of error representative of the cumulative positioning errors inherent to each of the control elements incorporated in the overall configuration. Thus, the inherent positioning error of the overall configuration is proportional to its mechanical complexity. Moreover, mechanically complex configurations like those exemplified by the prior art are potentially unreliable and/or expensive to maintain. Their accuracy is also subject to rapid deterioration caused by normal wear.
The invention overcomes the disadvantages aforesaid in a manner which maintains the capability to independently pivotally position the two backup rolls relative to the block, while eliminating one of the degrees of freedom inherent in the first basic configuration outlined above. The invention considerably simplifies the prior art structures, while eliminating many of their inherent disadvantages and yielding advantages not accomplished by the prior art.